Conventionally, there is known a differential unit for absorbing a difference in rotational speed between left and right wheels. The differential unit mainly includes a differential case, a pair of pinion gears which rotates together with the differential case (revolution), and a pair of side gears engaged with the pinion gears. The differential unit absorbs the difference in rotational speed between the left and right wheels by utilizing a fact that rotational speeds of the respective side gears change when the pinion gears rotate (rotation).
There is a differential unit including washers between a differential case and pinion gears. Such a differential unit is completed by inserting a dummy shaft into the pinion gears and the washers, and then replacing the dummy shaft with a pinion shaft (see Patent Literature 1, for example). This is because the pinion gears and the washers are supported in predetermined positions to facilitate insertion of the pinion shaft. However, this step has a problem that the washers fall off the dummy shaft. In particular, this step has a problem that the washers move due to vibration in a conveying step and the washers fall off the dummy shaft.